Sel 551 - Page 105

4.19
Date Code 20110408 Instruction Manual SEL-551 Relay
Setting the Relay
Settings Explanations
Example 1: Maximum Fault Current With an 80 A Instantaneous Setting
Maximum fault current in terms of primary CT and ANSI voltage rating,
burden in ohms, and X/R ratio is:
Equation 4.2
Equation 4.2 is an actual-value equation derived from Equation 1 above,
An SEL-551 phase instantaneous overcurrent element is to be set at 80 amps.
The relay will be used with a C400, 400:5 current transformer with a 0.50 W
ohm total burden. The X/R ratio is 20. Determine the maximum fault current
for dependable operation.
The burden is primarily from the CT windings and external leads to the
SEL-551 (the SEL-551 has a negligible burden):
Equation 4.3
Example 2: Minimum CT Rating With an 80 A Instantaneous Setting
CT rating in terms of maximum fault current, X/R ratio, ANSI rating, and
burden is:
Equation 4.4
With an 80 amp instantaneous setting, what is the minimum CT rating that can
be used when the maximum fault current is 40000 amps, X/R = 20, and the
burden is 0.50 Ω ohms?
where:
I
MAX
= the maximum primary fault current for line-to-line fault
CT
RATING
= the CT primary rating in amperes
Z
B
= the total CT secondary burden in ohms
ANSI = the ANSI voltage classification of CTs
Description Quantity
300 feet full-circuit run of #10 AWG (1.0 Ω /1000-ft) 0.30
CT winding of 80 turns at 0.0025 Ω/turn + 0.20
Total burden 0.50 Ω
I
MAX
262.5
1
X
R
----+
⎝⎠
⎛⎞
-------------------
ANSI
100 Z
B
•
----------------------
CT
RATING
• • =
I
MAX
262.5
1
X
R
----+
⎝⎠
⎛⎞
-------------------
ANSI
100 Z
B
•
----------------------
CT
RATING
• • =
262.5
120+()
--------------------
400
100 0.50Ω•
------------------------------
400• • =
40000 A=
CT
RATING
1
X
R
----+
⎝⎠
⎛⎞
262.5
-------------------
100
ANSI
---------------
I
MAX
Z
B
• • • =